As a mechanical-vibration detecting apparatus and a vibration-reduction control apparatus in a motor control system, there is an apparatus proposed in Japanese Patent Application No. 127859/94 filed by this applicant. In this prior art, a motor-containing mechanism system is separated into an equivalent rigid body system and a mechanical vibration system so that mechanical vibration is estimated at a high speed by an observer based on an equivalent rigid body system model to perform vibration reduction control.
In the prior art, however, a quadratic observer is constituted with the angular velocity and step-like torque disturbance of the equivalent rigid body system as state variables, so that not only an equivalent rigid body model and a proportional operation means but also an integrating operation means are required in order to constitute a mechanical-vibration detecting apparatus. Two parameters are required to be set for the observer and, furthermore, the phase of a mechanical vibration signal varies largely in accordance with the set-values of the parameters so that there arises a problem that it is difficult to adjust the parameters.